Assessing and Managing Methylmercury Risks Associated With Power Plant Mercury Emissions in the United States

Gail Charnley, PhD

In This Article

Exposure to Mercury From the Environment

Sources of Mercury

Mercury (Hg) is a naturally occurring element that is ubiquitous in the Earth's core, crust, soils, oceans, and atmosphere. Mercury can exist in elemental, inorganic, and organic forms, but almost all of the global mercury pool is elemental.[1] One of the organic forms, methylmercury, is the form of current toxicologic concern. Mercury can and does change its chemical form and move among different environmental media, with the global amount of mercury remaining the same, but our understanding of its environmental behavior is still evolving. Human activity has made substantial amounts of mercury bioavailable that would not be under natural conditions.

Trace amounts of mercury exist naturally in fossil fuels, such as coal and oil. When those fuels are burned to generate electricity, mercury is released from the fuels to the air. Of the approximately 48 tons of mercury released into the air annually as a result of human activity in the United States, electric power generation contributes about 40%.[2] Other sources of airborne mercury that are related to human activities include medical waste incineration, municipal solid waste incineration, mining, pulp and paper milling, and cement manufacturing. Regulation of emissions from waste incineration has reduced airborne mercury emissions from human activities in the United States by more than 45% since 1990.[2] Natural sources of mercury released into the air from its solid form include volcanoes, oceans, forest fires, and soils. Electricity generation contributes about 10% of mercury emissions in the United States when natural sources are included. Globally, all US anthropogenic sources contribute about 2% to 3%, and US electricity generation contributes about 1% of total mercury emissions into the air (see Figure 1).[3,4] Asia accounts for more than half of the mercury emitted globally into the air from human activities, and China's coal-fired power plants alone represent about 22% of those emissions.[3,4]

Figure 1.

Approximate extent to which global atmospheric mercury can be attributed to different sources (based on Pacyna and colleagues[3]).

Mercury is usually released into the environment in its inorganic forms. Mercury vapor can be transported globally, whereas other forms are generally deposited relatively close to their sources. Inorganic mercury deposited in sediment can be converted to organic methylmercury by microorganisms (a conversion that does not take place in the human body).[1] Because mercury in the atmosphere can originate globally, regionally, and locally, the extent to which individual sources contribute to environmental mercury is debated. The EPA estimates that about half of the mercury deposited in the United States from the atmosphere is from US sources, whereas the remainder is from global sources.[5] The Electric Power Research Institute estimates that anywhere from 20% to 80% of the mercury deposited in the United States comes from domestic sources, depending on location, with virtually all of the mercury deposited west of Minnesota and the Mississippi River coming from outside the United States.[6] Seigneur and coworkers[7] estimate that about 70% of the mercury deposited in the United States comes from non-US sources (both natural and anthropogenic).

Dietary Mercury Exposure

When mercury from the atmosphere gets into water bodies and is converted to methylmercury in sediment, it can enter the aquatic food chain, where it bioaccumulates in seafood. Large, predatory saltwater fish contain more methylmercury than smaller fish species due to accumulation effects at each increasing stage of the food chain.[8] Levels in freshwater fish also vary, with higher fish methylmercury concentrations occurring in water with higher levels of methylmercury contamination.

The primary source of human exposure to methylmercury is the consumption of fish, shellfish, and marine mammals. Because different species contain differing concentrations of mercury, exposure varies according to the types of seafood that are consumed. Mercury concentrations in fish vary by about 2 orders of magnitude, with herring containing about 0.01 ppm and shark more than 1 ppm, for example.[9] The EPA estimates that the population as a whole consumes an average of about 1.4 mcg methylmercury daily, whereas the more recent US Food and Drug Administration (FDA) Total Diet Survey data can be used to estimate a mean daily intake of about 0.8 mcg ( Table 1 ).[10] Some ethnic groups and subsistence or sports fishers can have higher dietary intakes of locally caught fish, with concomitantly higher methylmercury exposures than average if those fish are contaminated. More than 75% of the fish consumed in the United States is imported.[11] Farm-raised fish (about one half of US fish consumption) tend to contain very low levels of methylmercury because it is generally not present in their artificial feed.

Several studies have evaluated whether the source of methylmercury contamination in ocean fish is more likely to be naturally occurring or associated with human activities. One such study comparing 1971 and 1998 methylmercury concentrations in tuna caught near Hawaii found no differences, although the level of mercury in the atmosphere has almost tripled since 1971 (due to non-US human activities).[12] The study authors concluded that the source of methylmercury in tuna (and probably in other ocean fish) could be primarily natural, not human in origin, originating in deep water or sediment. Similarly, Miller and colleagues[13] found no difference in the mercury concentrations of museum tuna specimens caught between 1878 and 1909 and tuna caught in 1972. Greenfield and colleagues[14] reported no obvious increasing trend for mercury levels in the tissue of striped bass caught off 3 widely dispersed locations throughout the San Francisco Bay area between 1970 and 2000 despite increases in environmental levels during that period. In contrast, during the same period, reductions in pesticide levels were observed in the same fish, paralleling restrictions on pesticide use. Together with the observation that US power plants contribute about 1% of global mercury emissions, these findings suggest that reducing US mercury emissions would have little impact on mercury levels in ocean fish.

Wild freshwater fish comprise about 10% of average US fish consumption, and the extent to which their methylmercury levels are affected by deposition of mercury from nearby coal-fired power plants is debated.[15] Most of the mercury emitted from power plants is elemental mercury or is rapidly degraded to elemental mercury, which tends to remain in the atmosphere and be transported away from the source, entering the global mercury cycle. A much smaller proportion remains in a form that is more likely to be deposited close to the source. Studies show that methylmercury levels in freshwater fish vary considerably, even among fish taken from lakes in the same region with the same sources of mercury. Site-specific factors unrelated to mercury, such as nutrient levels and water chemistry, appear to play a substantial role in determining freshwater fish methylmercury levels.

A study in Florida evaluated the relationship between mercury emissions, deposition, and total mercury accumulation in fish at different locations.[16] No data were collected on mercury speciation. Results showed that between 1991 and 2000, regulation of mercury emissions reduced emissions by 93% and deposition via rain by at least 25%; at 3 of the 12 locations sampled, large-mouth bass showed an approximately 80% decline in mercury levels during that period, whereas bass from the other locations showed no change or increases in mercury content.[16] Although the nature of the sites sampled varied, reasons for the site-specificity of the relationship between declining mercury emissions and fish mercury levels are unknown. In another study, the EPA used the results of a fate and transport model to conclude that, for long-term steady-state conditions, reductions in fish-tissue concentrations are expected to track linearly with reductions in air deposition watershed loads.[17] That conclusion has not been validated by data, however, and the model does not take into account the various ways that site-specific characteristics can alter the relationship between deposition and fish-tissue concentrations. Together, these observations suggest that reducing mercury emissions can reduce fish methylmercury concentrations close to the source in some places but not others.

The potential relationship between power plant mercury emissions and methylmercury concentrations in locally caught fish is complex and poorly understood. Conclusions about the effectiveness of limiting local mercury emissions as a means of reducing local freshwater fish methylmercury levels should be postponed until studies of the impact of current efforts to limit emissions become available.

Body Burdens

Mercury is excreted into and accumulates in human hair, which can therefore be used as an indicator of longitudinal mercury exposure. Most of the mercury in hair is methylmercury.[1] Hair mercury analysis is sensitive to the possibility of mercury contamination both pre- and post collection.[1] Contaminated hair samples would lead to overestimates of exposure, potentially affecting dose-response characterization. The pharmacokinetic behavior of methylmercury and its relationship to toxicity has been reviewed by Clarkson.[18]

The US Centers for Disease Control and Prevention has measured the blood mercury levels of women and children as part of the National Health and Nutrition Examination Survey.[19,20] The most recent results (1999-2002) show that women of childbearing age[16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49] had a geometric mean blood mercury concentration of 0.92 parts per billion, with an upper 95th percentile concentration of 6.04 parts per billion. Children ages 1-5 years had a geometric mean blood mercury concentration of 0.33 parts per billion and an upper 95th percentile concentration of 2.21 parts per billion. Mercury levels have also been measured in umbilical cord blood, and the ratio between cord blood and maternal blood is about 1.7.[8,21] There are few data on methylmercury levels in breast milk, but the concentration of total mercury in human milk appears to correspond to about 8% of the concentration in whole blood.[22]


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